By applying the CCSD(T)//B3LYP method with 6-31 + G(d,p) basis sets, this work presents possible mechanisms of various fragmentation products found in the glycine mass spectrum. Imposing the criterion of low fragmentation energy enabled the m/z 17, 28, 29, 30, 31, 39, 41, 44, 45, and 74 fragments to be identified as NH3+, HNCH+, H2NCH+, H2NCH2+, H3NCH2+, NCCH+, H2NCCH+, NH2CO+, COOH+, and CH2CO2H+, respectively. This result indicates that the dominant fragmentation process arises from a C-alpha-C cleavage by abstracting COOH, which corresponds to the m/z 30 peak (NH2CH2+). This interpretation is consistent with experimental observations. The neutral species of the m/z 28 (HCNH+) peak is CH(OH)(2) rather than [CHO + H2O] because the corresponding critical energy of the latter is higher than that of the former. The intermediates in the reaction pathway by which NCCH+ and HNCH+ fragments are formed reveal that water may participate in the reaction and stabilize the intermediates.